It is known that rubber compositions generally are combined or "compounded" with various other materials before being cured and/or put into one. Some of these added materials improve the properties of the end product in service while others improve processing properties of the uncured compositions. In some instances, both effects may be achieved. It is also known that the various chemicals, pigments and other materials so used, both organic and inorganic, can interact in various ways to produce desirable or deleterious effects. For further discussions of rubber processing and materials used therein, see, for example, Encyclopedia of Polymer Science and Technology, Second Edition, published by John Wiley and Sons, New York (1970), particularly Vol. 12, page 280 and The Vanderbilt Rubber Handbook, R. T. Vanderbilt Company, Norwalk, Conn., 06855 (1968), particularly Sections 6, 7, 8, 9 and 11.
Vulcanizing agents, plasticizers, extenders, accelerators, fillers, pigments, etc. generally are incorporated into vulcanizable rubber compositions so that the rubber can be cured or vulcanized in a mold to form useful articles. It often is necessary to include processing aids in rubber compounds prior to molding and curing. These aids are primarily intended to improve the mixing of the ingredients of the rubber compound, the processability of the rubber, the mold or mill release properties of the rubber, tack and green strength without seriously adversely affecting the properties of the cured rubber.
Vulcanizing or curing agents used in vulcanizable rubbers generally are sulfur or sulfur-containing compounds or peroxide compositions. The rate of the vulcanization reaction generally is slow with many rubber materials unless an accelerator is incorporated into the vulcanizable mixture. A number of materials have been suggested and utilized for their accelerating effect. Such materials include metal oxides, for example, lead oxide, calcium oxide and magnesium oxide. Organic accelerators have found wide use in today's technology, and many of these are derivatives of aniline. A larger portion of the organic vulcanization accelerators which are in current use are derivatives of 2-mercaptobenzothiazole (MBT). One group of MBT derivatives which has found wide acceptance includes the N-derivatives of 2-benzothiazole sulfenamide. A number of such derivatives and their use as accelerators of vulcanization are described and discussed in Vol. 20 of the Encyclopedia of Chemical Technology, Kirk-Othmer editors, Second Edition, 1983, pp. 337-363. See also U.S. Pat. No. 2,367,827.
In order to minimize or eliminate premature curing of the rubber formulation (scorching), the vulcanizing agents and accelerators are added to the formulation just prior to the curing step. The other normally used rubber formulation additives are mixed with the base rubber compositions in, for example, a masterbatch operation, prior to contact with the sulfur and accelerator.
Carbon blacks are used in rubber formulations and vary widely as to their characteristics and combinations of characteristics. In rubber formulations, carbon black is used as a reinforcing filler. Many carbon blacks of the channel and furnace types with varying characteristics have been utilized because they impart varying desirable characteristics to the rubber. The formation of a secondary network structure in rubber stocks containing reinforcing carbon blacks leads to high hysteresis at low deformations. Various additives have been suggested in the art to reduce the hysteresis of such rubber stocks. Hysteresis is defined in the Dictionary of Rubber Technology, A. S. Craig, Philosophical Library Inc., New York, p. 80, as the difference between the energy input and energy output when rubber is deformed. The loss in energy is consumed in internal friction and results in heat buildup. Thus tires made of rubber exhibiting high hysteresis are characterized by high running temperatures.
As mentioned, the present invention relates to tires having low rolling loss. Since the tread portion of a tire is adapted to be ground contacting and occupies a considerable portion of the tire thereof, it is advantageous to use a tread composition which will produce a tread having desirable properties. Rubber compositions which will produce tires having a small hysteresis loss due to deformation of the tires when they are rolled have been used in the industry in attempts to obtain tires having decreased rolling resistance. Conventional tread compositions normally are composed of materials which tend to increase hysteresis loss in the resulting treads.
One of the difficulties of producing treads having decreased hysteresis loss is the potential resulting loss of other desirable properties such as braking performance wet and dry traction, and wear resistance. Thus, it is desirable to develop rubber formulations useful in making the treads of tires which reduce the hysteresis loss of the treads without impairing braking performance, traction, wear resistance and other desirable properties.
British Patent Application GB No. 2010850A describes the use of certain heterocyclic di-N-oxides as cross-linking agents for unsaturated polymers, especially rubbers. In general, the heterocyclic di-N-oxide compounds are defined as compounds whose structure comprises a 6-membered hetero-aromatic ring consisting of four carbon atoms and two nitrogen atoms in the 1,4-position of the ring, both nitrogen atoms being oxidized to N-oxide groups. A preferred example of such compounds listed in the British patent is represented as ##STR2## wherein the R groups R.sup.1, R.sup.2, R.sup.3 and R.sup.4 may be hydrogen, alkyl, aryl, alkoxy, aryloxy, alkaryl, aralkyl, acyl, alkylsulfonyl, arylsulfonyl, halogen or nitro. Two or more of the R.sup.1 -R.sup.4 groups may be linked together to form further ring systems and any of the groups or ring systems may be further substituted. In a specially preferred class of compounds described in the British patent as the above-identified compound has the two carbon atoms in the two and three-position of the ring fused to an aromatic ring. The patentees indicate that the cross-linked polymers obtained by the process of the invention are of higher molecular weight and possess better tensile properties than before cross-linking. Specific examples of heterocyclic di-N-oxides disclosed in the British patent include: pyrazine-1,4-di-N-oxide; quinoxaline-1,4-di-N-oxide; 2-acetyl-3-methyl quinoxaline-1,4-di-N-oxide; and 2,3-dimethyl quinoxaline-1,4-di-N-oxide. The cross-linked polymers described in the British patent are suggested as being useful as decorative or protective coatings and as ingredients for vehicle tires, hoses and other products.
U.S. Pat. No. 4,309,318 describes tread compositions for low rolling resistance tires comprising styrene-butadiene based rubber, carbon black and sulfur. Preferably the carbon black has an iodine adsorption number of 60-100 gI.sub.2 /kg and a DBP adsorption number of at least 119 cc/100 g.
European Published Patent Application No. 156,755 describes tires having sulfur-cured elastomeric tread compositions comprised of (a) medium vinyl polybutadiene, (b) 1,4-polyisoprene rubber, and (c) styrene/butadiene copolymer rubber. Tires constructed in this manner are reported to be characterized by improved rolling resistance without appreciable degradation of the wet and dry skid resistance and treadwear properties of the tires.
European Published Patent Application No. 159,469 relates to tire tread compounds comprising predominantly styrene-butadiene rubber reinforced with a special high structure (superfine) carbon black designated therein as N103. Tires made with such oil-extended tread formulations exhibit excellent abrasion resistance, traction and handling characteristics.
Rubber formulations useful for carcass and tread applications are described in U.S. Pat. No. 4,259,218. The formulations comprise a blend of medium vinyl polybutadiene and natural rubber reinforced with a carbon black having a highly negative tint residual.